Détails:One of the propeller blades failed in flight due to fatigue, causing violent vibration. The engine mount then failed causing the powerplant to rotate to the left. The prop cut through the spar of the port lower wing, causing the wing box to collapse. The aircraft entered a spin and crashed into the ground near the Yuzhnyy collective farm.The aircraft operated on a test flight with a mechanic on board. The propeller had been fitted the day before.